The process that has produced a nonuniform distribution of matter throughout the universe, i.e. large-scale structure, such as galaxies. In general, ‘clumpiness’ occurs when the matter is not homogeneous, i.e. the gravitational attraction between particles of matter ceases to be uniform. This can occur as a result of quantum fluctuations in the early universe associated with inflation and the Jeans instability in an expanding universe. There is some evidence for this hypothesis, based on measurements from COBE, WMAP, and the Planck satellite. An alternative mechanism for structure formation is based on cosmic strings but observations are not in accord with the predictions of this theory. A complete quantitative theory of structure formation does not exist at present.